A PGC reader emailed NOA about whether GBA would be available the 11th or the 13th and got a really good response straight from the horse's mouth, which explains things nicely:
Here is the email:
Hey Nintendo,
A lot of people are saying that their local Toys 'R Us, Best Buy, WalMart, and many other stores have posted signs in their store stating that NoA has changed the US GBA Release date to June 13, 2001. Can you clear this up for me? Thanks.
Here is NOA's response:
Hi there!
The official release date for the Game Boy Advance is June 11th, but the "release date" and the "street date" may be two different things. If stores are advertising that they'll have it on June 13th, then you may have to wait until then to play. Since it takes a day or two for our products to ship, this may explain the difference in date. I hope this clarifies things a little. Enjoy!
Stephen Scott
Nintendo of America Inc.